1. Field of the Invention
This invention relates to test apparatus and, more particularly, to testing packaged electronic devices.
2. Description of the Related Art
Time domain reflectometry (TDR) is an established technology for testing electrical cables. By transmitting a short electrical pulse into a cable and examining the electrical signatures of reflections, a test apparatus may detect the presence of open circuit and short circuit faults in the cable. In addition to detecting the presence of faults, it is possible to determine the distance along the length of the cable at which the fault is located by measuring the time interval between the transmission of a pulse and reception of a corresponding reflected pulse.
In order to measure distance via TDR, it is generally necessary to calibrate the velocity of propagation of pulses in the cable or other electrically conducting medium under test. An electrically conducting element and its corresponding return conducting path may be said to form a transmission line having a characteristic impedance. The characteristic impedance is a function of the geometry between the conducting element and its return path and the effective dielectric constant of the material separating them. An electrical pulse propagating in the transmission line will reflect, at least partially, from any point at which there is a change in the characteristic impedance. In some cases, it is possible to calculate the characteristic impedance knowing the geometry of the transmission line and the type of material separating the signal and return paths. In other cases, it is possible to measure the velocity of propagation by measuring the length of a known, defect-free sample of the transmission line to be tested and the time between the transmission of a pulse and a reflection from the end of the transmission line.
In order to accommodate a variety of cable lengths, materials, and geometries, it is often necessary to adjust the characteristics of electrical pulses transmitted into a transmission line. A longer-width pulse may inject more energy into the transmission line, propagate further, and produce a stronger reflection than a shorter-width pulse. A higher amplitude pulse may also produce a stronger return signal, improving the signal-to-noise ratio of the measurement. The faster the rise-time, the finer the resolution obtainable. Accordingly, adjustments are typically made to the pulse width and amplitude in order to obtain a suitably strong signal from a transmission line under test. In addition, the resolution with which a length of transmission line may be measured is typically dependent upon the rise-time of the pulse transmitted into the transmission line under test. Consequently, it may be desirable to use as fast a rise time as is economically practical for the desired measurement.
A TDR measurement typically requires test equipment such as a wide-bandwidth sampling oscilloscope capable of injecting a pulse into the device under test. Commercially available TDR equipment generally comprises a wide-bandwidth oscilloscope coupled to specialized add-on TDR modules and analysis software tools, available from numerous vendors. For example, general purpose wide-bandwidth oscilloscopes such as the Agilent® 86100B equipped with a 54754A TDR pulse generator module and N1930A physical layer test system software or the Tektronix® 8000 Series oscilloscope with a TDR-equipped 80E04 electrical sampling module may be employed to perform TDR measurements. Also, automatic test equipment (ATE) such as the Teradyne® Integra J750 test platform with the Femto® 2000 Time Interval Analyzer installed is capable of performing TDR measurements. Other vendors, such as Picosecond Pulse Labs® offer components that, with applicable hardware and software options, may be used to perform TDR measurements.